3.1 Rigid containers

Rigid containers include glass and plastic bottles and jars,
cans, pottery, wood, boxes, drums, tins, plastic pots and tubes. They all, to
varying degrees, give physical protection to the food inside that is not
provided by flexible packaging. While most rigid containers are strong, they
are, because of the amount of material used in their production, more expensive
than flexible packaging. Some types of rigid packaging have the advantage of
providing a perfect airtight hermetic seal.

3.1.1 Glass

Glass is made by heating a mixture of sand, soda ash and lime
usually with a proportion (up to 30%) of broken glass or gullet to about 1500
°C until it melts into thick liquid mass. The molten glass is blown into
moulds, in two stages, to make bottles and jars which are then cooled under
carefully controlled conditions to prevent weaknesses and breakage. As the raw
materials for making glass are cheap and available in most countries glass
factories are to be found worldwide. Glassmaking is however a very energy
intensive industry.

Glass packaging manufacture is only economically possible at
large scale. As the moulds are very expensive, only very large food companies
can afford to have their own moulds in the glass factory to produce their own
special bottles. Various standard colors are made including clear, green or
brown depending upon the protection needed from light.

If a glass factory exists in a country then bottles and jars can
be a good option for small scale food manufacturers. If the bottles have to be
imported, however, they tend to be very expensive compared with alternatives
such as plastic due to their extra weight. Many small manufacturers start
packaging with second hand glass. As the enterprise expands however it is found
that new bottles have to be bought. The high cost and poor availability of new
bottles then becomes a major concern. Many producers finally turn to
alternatives, such as plastic packaging, or accept total reliance on second hand
containers. In many countries a sub-industry exists to collect, wash, sort and
sell used bottles and jars.

As will be described later, use of the correct type of lid is
vitally important. Once again however many small producers find obtaining lids a
major problem as the minimum orders required by the suppliers are high and few
glass manufacturers also supply lids. The names used for the various parts of a
glass container are shown in Figure 3.1

Figure

Glass has several advantages and disadvantages as a packaging
material as shown in Table 3.1.

Advantages

Disadvantages

Chemically inert (no reaction

Breaks with rapid

with any food)

temperature change

Strong, can resist internal

Fragile, poor shock resistance

pressure and weight

Can be re-used and re-cycled

is heavy

In-plant breakage carries

danger of splinters in food

Impermeable to gases,

aromas and moisture

Can give protection against

light

Barrier to micro-organisms,

insects etc

Can be heat-sterilised

Good product display in clear

glass

Long shelf-life possible

High customer appeal and

acceptability

Good protection against

physical damage

Table 3-1: Advantages and disadvantages of glass

Physical properties

The main physical advantage of glass is its inertness and
impermeability. Processors do not need to worry about the type of glass needed
as they do with cans and plastics which can react with certain types of foods.
Glass has the additional major advantage of being re-usable, re-cyclable and not
damaging to the environment.

Products that are affected by light or have a long shelf-life
benefit from packing in coloured bottles. While glass is fragile to shock it is
strong in terms of bearing weight so stacking on pallets is possible. Protection
is needed against shock by the bottles knocking each other or being dropped. For
this reason glass bottles are usually put into cardboard outer boxes with
dividers or card layers.

By the nature of the way they are made, glass bottles can var,
in wall thickness and also in weight. In many developing country glass factories
such variations are greater than international accepted norms due to the
reluctance of the producer to replace expensive worn moulds. This is very
important as it can give false data on the true fill weight or net weight. This
is discussed more fully later in the section on specific quality control aspects
for glass.

Most glass containers are made with a wall thickness related to
their size but carbonated dunks bottles, which have to withstand high internal
pressures, are made of thicker glass. Table 3.2 shows typical data on the some
common types of glassware used for foods selected from the very wide range
available.

Container

Height

Diameter

Weight

Volume

Closure

mm

mm

mm

g

ml

Bottles

Round

188.7

55.3

280

192

Crown

mineral

Round

165.0

65.5

187

265

Crown

mineral

Round

254.4

75.5

454

550

Crown

mineral

Round

298.2

77.8

500

700

ROPP

liquor

Square

268.9

71.8

490

700

ROPP

liquor

Round

270.9

77.7

440

750

RO

cordial

Winw

285.0

81.1

440

750

Cork

Jars

Jam jar

120.9

70.6

180

1 lb/366*

Push-on

Jam jar

121.0

72.5

185

1 1b/375*

TO

*Jam jars are commonly measured in Ibs

Codes

RO Roll-onTO Twist-offROPP Roll-on Pilfer Proof

Table 3-2: Common types of glassware

The shape of the bottle or jar is also important as some shapes
are weaker than others and so need greater protection. A round bottle is about 4
times as strong as a square one with rounded comers and 10 times as strong as a
square one with sharp corners. Unless there are important reasons related to
marketing, the use of simple round bottles is thus recommended to reduce
breakage and shipping container costs.

Preparation of glassware for filling

All glass packaging, whether new or second hand requires
cleaning before use. In the case of new glass simple washing in clean water is
all that is required. Much greater care is required when using secondhand or
returnable bottles and the following steps are recommended;

- visual inspection for cracks, chips, etc.- containers
should be smelt to make sure they have not been used to hold a substance that
might be poisonous or taint the food being packed,- removing labels by
soaking in 1% caustic soda and detergent,- thorough washing, using bottle
brushes,- rinsing in clean water,- if prepared re-used glass is to be
held in store until required it must be re-washed prior to use.

As has been pointed out glass breaks if rapidly heated or cooled
so bottles must be carefully heated before hot filling with product and then
carefully cooled. It is usual to pre-sterilize glass by either pre-heating in
water and holding at 100 °C for 10 minutes or steam sterilizing. Steam
sterilizing has the advantage that any weak bottles are more likely to break at
this time rather than when they have been filled with a hot product. This
reduces the risk of contaminating the food or wasting the product. Steam
sterilizing of bottles (Figure 3-3) must be carried out in an area that prevents
any splinters from bottles that may break entering the product so injuring the
consumer. This is discussed further in Chapters 4 and 5.

Figure

Food packaged in glass containers can have a very long shelf
life provided that the food has been properly processed before packaging, no
contamination occurs at the filling stage and that the container is properly
closed with a lid or seal. It should be remembered that the pack is only as good
as the closure. Recommended shelf-lives vary but are usually 6 to 12 months not
because the product actually deteriorates, but because over time there is a
gradual loss of colour and flavour. Some foods, wines and spirits for example
actually improve during prolonged storage and it is not unusual for a bottle of
wine to be drunk ten or more years after packing.

In addition, in some countries vegetables and meats are packed
in glass jars which, after closing, are heat treated under pressure in the same
way as canned foods. The use of glass in this way cannot be recommended to small
producers due to the health risk it carries.

When hot filling, the sterilization by hot water or steam
described earlier ensures that the container is clean and 'sterile' when filled.
In addition the hot filling operation at 80°C or above means that the
product is also 'sterile'.

It has been found that when hot filling products such as fruit
juice it is good practice to lay the capped filled bottles on their side for
about ten minutes before cooling. This allows a vacuum to form in the bottle and
the cap to 'tighten down' onto the bottle neck. Experience has shown that this
laying on the side dramatically reduces post-filling contamination because it
removes the possibility of small amounts of air being sucked into the bottle
until the neck seal is perfectly formed.

After hot filling, careful cooling must take place as a hot
bottle put in cold water will probably shatter. The packs can be laid on their
side to cool, which takes time, may result in flavour changes and occupies
valuable factory space. Alternatively a simple cooler shown in Figure 3-4 can be
made which gives controlled cooling. In this cooler cold water enters at the
deep end of the trough and overflows from the shallow end. The hot bottles enter
at the shallow end and are taken out from the deep end. What happens in practice
is that the temperature is cool at the deep end and becomes warmer and warmer
towards the shallow end due to the heat being taken from the bottles as they
cool down. At the start of the day the whole tank needs to be filled with hot
water to prevent damage to the first few bottles placed in the cooler.

Figure

When cold filling there is much more risk of contamination and
so cold filled glass must be thoroughly washed in water containing chlorine
(about 5 to 10 drops of bleach per gallon of water). In some cases, particularly
when bottling wines and vinegars, bleach can cause off-flavours and the use of
sodium metabisulphite is recommended (one teaspoon per gallon of water).

Glass bottles may be filled by hand, from gravity fillers,
piston fillers or vacuum fillers, either manual or automatic depending on the
product and scale of operation. Suitable fillers are described in Chapter 4.

Sealing or closing jars and bottles

The type of cap or closure and the method of application depend
on which of five main types of container is being used. Closures are mainly made
from metal or plastic although corks still find wide application for wines.
Whatever the type of closure used:

- no part of the closure should affect or be affected by the
food in the container,- it must seal properly and remain sealed for the
shelf-life of the food,- it should be convenient for the customer and if the
product is one that is not all used at once, it should be able to be
re-closed,- it must meet the increasing demand of both customers and traders
for being tamperproof.

When selecting caps for a particular combination of bottle and
product it is very important to take advice from the supplier regarding the
suitability of the closure for the intended use. The range of alternatives in
terms of lacquers, finishes and linings is great. It appears that no written
data is produced recommending a particular lacquer or coating for a particular
food. In practice the best alternative is found by packing the food and testing
for any interaction between closure and contents by visual inspection.

Metal caps are made from tinned steel or aluminium. Being strong
they are very suitable when the bottle has a vacuum formed after hot filling or
when it is under pressure. Steel caps are the strongest. Both types can be
lacquered to give added resistance to reacting with the product.

Plastic caps are made mainly from polypropylene (OPP) and
polythene, both low and high density (LDPE and HDPE). The gas barrier properties
of LDPE and HDPE are lower than OPP. As they are supplied moulded with a
pre-formed thread, plastic caps have to be very accurately matched to the type
of bottle or jar being used. Variations in glass neck and thread sizes may cause
sealing problems that will not occur when using crown, RO and ROPP caps.

Almost all caps contain a lining material which has two
functions, first to provide a soft 'cushion' so that the cap will tighten down
to the bottle neck and secondly to reduce contact of the food with the cap.
Liners can be plastic (usually PVC), plastic coated paperboard, waxed paperboard
etc.

Some caps such as screw-on and twist-on twist-off can be applied
by hand. Crown caps, push-on type caps, ROPP caps, plastic hinge-open and
snap-shut and corks need machines to put them onto the bottle. Small manual
equipment is available for this. All caps can of course be applied by semi- and
fully automatic machines but the small and medium-scale food producer is likely
to use only manual methods.

Crown caps are applied by a combination of downwards pressure
and crimping the skirt and small flutes over the lip on the bottle neck finish,
so locking it on. They always have a liner. As crowns cannot be put back on by
the customer they are only suitable for products that are opened and used at one
time. For larger-scale operations semi- and fully automatic versions are
available but beyond the scope of this publication.

Small manual machines are available for push-on caps commonly
used on jars of jam - which crimp the rim of the cap around the bottle neck
finish. The lowest-cost machine simply crimps the cap edge while the larger
version is fitted with fingers that make small indentations in the cap edge, so
giving a firmer seal.

Roll on caps (RO) are made of aluminium and supplied unthreaded
as a small cup. The action of the capping machine forces the cap wall into the
thread of the bottle, then forming a thread in the cap. RO closures are often
supplied with a perforation along the bottom edge which breaks when the cap is
unscrewed. Such caps are pilfer proof and thus called roll-on pilfer proof
(ROPP). They are most commonly fitted to high-value food products where there is
a risk of pilfering or adulteration.

As far as is known no commercially available very cheap manual
RO or ROPP capping machines exist. Drawings are however available from ITDG,
United Kingdom of such a machine that has been developed in Sri Lanka.

Corkers

Corks are mainly used to close wine bottles by pressing them
into the neck under considerable pressure while at the same time squeezing them,
to reduce the diameter so that they will enter the bottle neck. The corks are
wetted before use so that they will slide more easily into the bottle neck. Once
inside the neck the cork then expands to give a tight fit. As corks are natural
materials and so may well be contaminated with micro-organisms it is recommended
that they are soaked in a solution of sodium metabisulphite (approx 1 teaspoon
to the gallon).

After corking it is widely recommended that bottles be stored
laying on their sides. This prevents the corks drying out and shrinking which
would increase the risk of external contamination.

Plastic hinge-open snap-shut closures

In some countries it may be possible to obtain closures of this
type which are becoming increasingly common for liquid products that are opened
and closed several times in use. Common applications include cooking oils,
sauces and fruit toppings. Simple hand-operated presses are available to fit
this type of closure.

Other tamperproof systems

Instead of fitting tamperproof caps it is possible to fit
several types of sleeve over the bottle cap that will show if the bottle has
been interfered with. Typical products are plastic shrink sleeves and aluminium
foil capsules. Two types of plastic capsule are used. One type is supplied wet
in tins and is simply slid over the bottle neck. As it dries it shrinks tightly
around the neck. The other type is heat shrunk in a small electrically heated
cylinder. At the small scale, aluminium capsules are applied with a simple
push-down crimping machine.

All the above capping machines are easy to use and require
little maintenance. They are suitable for small producers with production rates
from a few hundred to several thousand packs per day. In developing countries it
is unlikely that semi- or fully automatic capping machines would be
appropriate except in large plants. The use of several small hand-operated
machines would be more economical.

Bulk transport of foods packed in glass

As has been mentioned glass is strong under compression so
finished goods can be piled in boxes, onto pallets or stacked. Glass will break
however if subjected to shock. Great care is needed not to drop full cases and
avoid one bottle banging against the next by using cases with card dividers. The
use and design of suitable boxes is described in detail in Chapter 3.1.6.

Quality control

One important quality control measure when using glass is the
variation in weight of the empty packs which distorts the net weight when full
packs are checked. General methods of controlling net weight are discussed in
Chapter 6.4.3. In the special case of glass however, a random sample (approx 1
in 50 containers) should be taken from each delivery. Sampling should be
scattered, not all from one case. The sample is then individually weighed and
the net weight calculated using the heaviest container. Thus: required filled
weight = Weight of heaviest bottle + net weight of product

In addition samples should be kept to make sure that cap
corrosion does not occur and that good seals are maintained for the shelf-life.
In hot filled packs this may involve checking for the maintenance of internal
vacuum. If automatic fillers and cappers are to be used, then variations in
height and diameter of glassware may become very important but production at
this scale is outside the scope of this publication.

As described under Quality Control, (Chapter 6.4) defects are
commonly divided into critical, major and minor defects. In the case of glass:

Pottery is one of the most ancient forms of traditional
packaging. Pottery wine and oil jars have been used for thousands of years.
Hundreds of yeas ago crude sugar was crystallised in pots, a stage known as
potting. Potting later was used to describe the method of preserving 'potted
meats' in clay containers. Although pottery containers have now been largely
replaced by other materials for commercial food packaging they still are widely
used in some countries for certain products, for example cooking oil, tomato
paste and gun They also find application when packing high value, luxury foods.
In Europe, for example, very expensive marmalades, meat pastes and cheeses may
be bought in glazed pots. The use of pottery for the small producer will thus
fall into two areas:

- as a low-cost, locally obtainable alternative to glass,
etc.,- to pack high-value foods for the richer customer or perhaps tourists.

Pottery containers are made from clays either by hand or with
the use of moulds. Hand-made pots vary considerably in size and shape while
moulded ware is far more standard and thus more suitable for routine food
packaging. After production the pot has to be baked or fired in a kiln at high
temperatures, between 600 and 1250°C. The appearance and properties of the
final product depend upon the type of clay used, the firing temperature and
whether or not the pot is glazed. Ordinary clays fired at low temperatures yield
earthenware. Other clay types, fired at higher temperatures produces stoneware
while the use of special clays and very high temperatures yields porcelain. The
fired pot may, if required, be dipped in a glaze and returned to the kiln where
the glaze melts to a glassy coating. Both external and internal glazing can be
applied. When earthenware is glazed, the glaze does not bond into the pot but
essentially sticks to the surface. As the pot cools such a glaze often 'crazes'
and the tiny cracksso produced mean that an incomplete impervious glaze coating
forms. When stoneware is glazed the glaze bonds into the clay and a far more
perfect protection results.

As the moulds for pottery containers are cheap to make it is
possible, unlike glass, to have special packaging made. An The basic properties
of both unglazed and glazed pottery are shown in Table 3-3.

Earthenware

Glazed

Stoneware

earthenware

Chemical

Reasonably

Very resistant

Extremely

properties

resistant to

if glaze not

resistant

chemical

crazed

attack

Permeability

Very

Low if glaze

Impermeable

to moisture

permeable

not grazed

and gases

Pack product

Can react

Little if well

None

interaction

with very acid

glazed with

foods

correct glaze

Operating

Good resistance. Can break if not warmed before hot filling

temperature

Weight

Heavy, has to be made thicker than glass to give equal strength

Strength

Very easily

Stronger than earthenware but breaks if

broken if

knocked

knocked

All are strong under load

Table 3-3: Properties of pottery packaging

Glazes

Great care must be taken if considering the use of glazed
pottery for food packaging that the glaze will not react with the food. This
becomes even more crucial if even slightly acid foodstuffs such as honey or
yoghurt are involved. Many glazes contain chemical salts of heavy metals which
are toxic. The main problem lies with lead.

Lead glazes are widely used on pottery since they are cheap and
easy to use. Other heavy metal glazes, which are generally highly colored are
used for decoration and so unlikely to be encountered on the inside of pottery
being used for food packaging. The food producer must make sure from the pottery
supplier that lead glaze is not used. The seriousness of the problem has been
highlighted from work in Central America where people traditionally use lead
glazed bowls and cups for food. Changes in diet, particularly children drinking
acid fizzy drinks, is showing up as increased lead in the body with resulting
chances of impaired brain development. Simple chemical tests exist for checking
for lead and any food manufacturer with doubts over the suitability of a glaze
is advised to have a container tested in a local laboratory.

Packaging applications for pottery containers

Pottery is still widely used all over the world for the
traditional packaging and storage of foods such as grains, pulses, wines, honey,
pickles, yoghurts and dried foods. Such traditional uses are outside the scope
of this publication which is concerned with the use of packaging materials for
commercial production.

It is almost, if not totally, impossible to hermetically seal
pottery containers due to the variations that occur in the neck diameter and
shape (slight ovality). For this reason their use is limited to products that
are either very stable, such as honey, or have a short shelf-life such as
yoghurts and soft cheeses.

In cases where pottery is being used for products aimed at
high-value markets, that is to say more for their visual appeal than barrier
properties, dry foods that would tend to absorb moisture can be packed in
plastic bags inside the outer pottery pot.

Product

Comments

Honey

Very stable, long shelf-life in

glazed pots, needs sealing to

keep out insects such as ants

Yoghurt and soft cheeses

Packed in shallow

earthenware bowls. Short

shelf-life. Needs covering ie

with paper tied round neck to

protect from dust and insects.

Must be very well cleaned if

re-used as food is absorbed

into the earthenware.

Solid block sugar (for

Very stable, pot needs to be

example gur)

broken to remove the product.

Spices, teas, herbs

Inner plastic liner needed in

humid climates, should be

sealed

Jams and jellies

Stable products with long

shelf life in glazed pots, must

be sealed

Table 3-4: Foods suitable for packaging in pottery containers

Using pottery containers

When using pottery packaging the same basic precautions apply as
for glass. Incoming pots should be inspected and any showing damage rejected.
Pottery pots are likely to be more dusty than glass due to the conditions they
are made in and the rougher surface. Thorough washing in clean water is
essential. The pots should be turned upside down and allowed to dry before
filling.

If hot filling is planned, for example with jam, pottery pots
may break during filling. Pre-heating in an oven is recommended. Pottery
containers are not usually heat processed. In practically all cases the small
producer wilt hand-fill the containers although small volumetric piston fillers,
as described in Chapter 4, can be used. Indeed, the use of volumetric filling,
even using a simple measuring jug, is recommended in view of the considerable
weight and size variations that occur in pottery containers.

Sealing

As has been mentioned it is almost impossible to hermetically
seal pottery containers. However, the following methods are commonly used to
produce an acceptable seal:

- Use of a cork bung. The seal can be improved by running
sealing wax around the bung edge.- Use of a pottery insert and a disc of
polythene.- Waxed paper or polythene held on with a rubber band or string.

Examples are shown in Figure 3-12. It is essential to keep
filled pots vertical as all the above seals are likely to leak if the pot is
turned upside down.

Figure

Shipping containers

Pottery, like glass, is easily broken. Careful packing in outer
boxes in the same way as glass jars is thus recommended. Larger pots, of the
type used for solid sugars or bulk distribution, are often packed in hand-made
wooden boxes lined with soft material like dry grass to absorb any shocks in
transport

Skills required

No special skills are required when packing in pottery except
perhaps learning to seal effectively with wax.

Quality control

There are several important quality control checks needed when
using pottery. First considerable size (volume) and weight variations may exist
from pot to pot or delivery to delivery. Samples should be taken and checked for
volume and weight on delivery. Samples of filled containers need to be taken and
checked for net weight more frequently than when using glass. If volumetric
filling is not used, it may be necessary to fill each container on a scale so
ensuring that the correct net weight is obtained.

The food producer must make sure that only safe, nonlead, glazes
are used by the supplier. Re-check periodically as the potter may change his
glaze.

If the pots are made of earthenware and re-used then great
attention must be paid to thorough washing and cleaning as earthenware, being
porous, will absorb food into the structure of the pot. This, due to
microbiological grown, can make the food deteriorate.

Pottery containers have certain advantages for small producers,
particularly those living in very isolated areas where alternatives may be hard
to obtain Indeed it is in such areas that the skills of the potter are most
likely to have survived

3.1.3 Metal containers

Metal containers commonly used in the food industry include
steel drums, tins with push-on or screw-on closures, sanitary cans (the 'tin'
can), composite cans (usually a combination of paper board and steel), aerosols,
aluminium cans and aluminium foil made into dishes, etc. The level of technology
involved in filling into aluminium cans (used for beers and carbonated
beverages) is high and as generally it is only applicable to large production
units will be only briefly covered. Aerosols are beyond the scope of this
publication.

Sanitary cans

Cans and glass are still perhaps the most common rigid
containers used for packaging and preserving food. While almost any food,
including dried goods, can be canned the most common applications are to fruit
juices, fruit in syrup, tomatoes, meats, fish and vegetables.

The processing methods required to can acid foods, such as
fruit, are very different from those needed to can low-acid foods safely, such
as vegetables and meats. Acid food products only require heating to temperatures
below 100°C in order to inactivate naturally occurring enzymes and destroy
most micro-organisms present Low-acid foods on the other hand need to be heated
in pressure vessels, called retorts, at 121°C for a pre-determined time
based on the product and size of can being processed. Cooling, under pressure
from compressed air, then has to take place in the retort

Equipment costs (steam boiler, retort, compressor) for low-acid
food canning are high and considerable technical skills and knowledge are
required to safely can low-acid products. Errors can cause severe food poisoning
or even death. It is strongly recommended that small and medium-scale food
manufacturers should not attempt to can low-acid foods such as vegetables, soups
or meats unless they have, in house, the necessary expertise and laboratory
facilities to make sure that production errors do not occur. The reasons for
this, and the food poisoning dangers that exist have been described in Chapter
2. This chapter has been written with only the canning of acid foods in mind.

The can has distinct advantages over glass which include:

- good heat transmission,- not subject to thermal shock so
rapid heating and cooling are possible,- lighter in weight,- not subject
to breaking,- little or no interaction between the food and can occurs
provided the correct type of can is selected,- resistant to physical damage.

They are also totally impervious to light and air. The main
disadvantage of cans is, of course, that the contents cannot be seen by the
purchaser.

Canning may be a good option for small and medium-scale
producers, particularly when a can-making facility exists in country. It should
also be remembered that cans are considerably lighter than glass containers so
transport costs can be lower. Cans are still an expensive option when compared
to alternatives such as plastics.

Can manufacture

There are three main methods of making cans. The most common
produces the traditional three-piece sanitary can which consists of a body and
two end pieces that are joined together to provide a hermetic or perfect seal.
While most commonly used for foods that are heat processed they also find
application in packaging powders, syrups, etc. that are not heat-processed. The
most common shape is a round cylinder but square and oval flat cans are used,
particularly for fish processing. The other two methods which produce a
two-piece can (integral body and base plus a lid) have become increasingly
common in recent years. Two-piece cans require less metal and thus are lighter
and cheaper.

Three-piece cans

Cans are most commonly made from thin sheets of steel that have
been electrolytically coated with tin on both sides. The type of steel used
depends on the corrosion resistance needed for the particular product to be
canned. The most resistant grade is called Type Land is used for strongly
corrosive foods such as apple juice, prunes, cherries and pickles.

Type MR steel is less resistant to attack and is used for mildly
acid products such as apricots, peaches and grapefruit as well as low-acid foods
like peas, corn, meats and fish Type MC is used for the low-acid foods mentioned
previously.

The tin layer is 0.1 to 0.3 mm thick (2.8 to 11.2
g/m2). The layer thickness required depends on how corrosive the food
being canned is. Thicker tin layers are needed for high-acid foods. The tin
layer may be of equal thickness on both sides of the plate or thicker on one
side. The sheets of tinned steel are coated with a lacquer on the 'inside' face.

Lacquer is used to:

- prevent taste changes that might occur from traces of metal
that dissolve in the food,- prevent discolouration of the inside of the can
especially in foods rich in sulphur such as fish and meat,- prevent
discolouration of the product.

Lacquers are often described fruit juice, meat or fish grade.
The actual detail of the composition, thickness, etc., of these lacquers is
beyond the scope of this publication. The most common lacquers include:

- Oleoresin lacquer now being replaced by epoxyphenolics. These
have poor resistance to attack by sulphur. R or fruit enamels have resistance to
staining by fruit pigments of the type found in berry juices. C enamels are used
when packing high-protein foods such as corn, peas and poultry.- Vinyl
lacquers have good adhesion and flexibility but do not resist high-temperature
sterilization well. Often used as second layers for canned beer, wine and
carbonated beverages as well as dry foods.- Phenolic lacquers have very good
chemical stability and low permeability especially against sulphide. Used for
fish and meat products.- Acrylic lacquers have good color retention and high
heat resistance.- Epoxy-phenolic lacquers, the most comonly used type.
Resistant to acids, good flexibility and high heat resistance. A wide range is
available to cover different uses such as fruits, vegetables, meats and fish

The choice of the correct lacquer is of great importance and
readers considering canning are strongly recommended to consult specialists in
the can supply industry regarding the best coating for the food to be processed.

Can lids have a ring of flexible sealing material around the rim
which is compressed in the canning machine to give a perfect seal.

Figure

Three-piece cans are supplied to the user in two forms:

- with the base joined to the can body by the can manufacturer
and sent to the food manufacturer together with the lid, or- as a
'flattened' or 'collapsed' can with the body cylinder flattened into an oval
shape and supplied with loose bases and lids

Flattened cans are cheaper to transport as more cans are packed
into each cubic metre. The user however has to invest in can-reforming machines
and incur additional labour costs in attaching both the can base and lid.

Two-piece aluminium cans are commonly used for beers and
carbonated drinks. The technology used is high and costly. Recently, a
small-scale beverage filling/carbonating/closing system has been developed. This
unit can produce up to 5750 cans per day and can also accommodate bottles. It is
of low cost compared to existing alternatives.

The production of cans involves high technology and large
outputs. This is particularly true of the two-piece can where outputs of at
least 150 million cans per year are needed for economical production.

Can sizes

While a wide range of can sizes exist with capacities between 71
ml and 10200 ml most foods are packed into cylindrical cans with a capacity of
140 to 900 ml. When ordering cans it should be remembered that in the United
States and Imperial systems the first digit relates to the number of inches and
the second digits the number of 1/16 of an inch Can sizes are always expressed
as diameter x height. Thus a can 307 x 409 is 3 7/16" by 4 9/16". It should be
noted that in the United Kingdom and Europe metric sizes are increasingly
replacing imperial sizes.

When placing orders canners must ensure that the correct size of
can for the chuck size of their can sealing machine is ordered.

Reforming and closing (or seaming) of sanitary cans

As has been mentioned previously cans will be delivered to the
food processor either 'erected' (with the base fitted by the supplier) or
'Battened' (with the body as a flattened cylinder). If flattened cans are used
the canner will need three machines in order to erect, flange and seal (or seam)
the cans:

- A can-body reforming machine which re-forms the flattened body
into a perfect cylinder.- A body flanger which bends over the ends of the
cylinder to form a flange into which the lid and base are sealed.- A seaming
machine which seals the bases and lids to the can body to make what is known as
a double seam shown in Figure 3-14. Seamers go through two operations by means
of two rollers. The first operation roller rolls the cover hook around the body
hook and the second operation roller tightens the two hooks to provide a double
seam.

Figure

If erected cans are bought, only one machine, the seamer, is
required to seam the lid to the body.

The choice for the processor between using erected or flattened
cans will greatly depend on local circumstances. Broadly, for the flattened can
it can be said that:

- they are cheeper,- transport costs are lower,-
equipment costs are higher,- higher operator skills are needed, since three
mechanical steps are involved,- labour costs are higher.

The decision thus involves balancing the first two advantages
above in financial terms against the latter three disadvantages largely on the
basis of the level of production.

Washing cans

Cans received from the supplier must be washed prior to filling
as shown in Figure 3-16. Hot water is sprayed into the can which is laying on
its side. As the cans roll forward to the filling point they tip half upside
down to allow any water to drain away.

Figure

Figure

Filling of cans

While automatic rotary or carousel fillers are used in large
canneries hand-filling is the usual method for small and medium producers. If
liquids, such as fruit juices are being packed normal filling systems involve
jugs, piston fillers or simple gravity fillers. When products such as fruit in
syrup are being produced the fruit should be packed into the can first and then
topped up with hot syrup. In order to facilitate further processing, juices and
top-up syrups are usually filled into the can at temperatures of about
80°C.

It is most important that the can is not filled to the top and
that a 'headspace' of 0.3 to 0.5 cms is left. The simple device shown in Figure
3-17 will greatly assist in maintaining a standard headspace.

Exhausting

Before sealing the can, air present in the headspace is removed
from the container by exhausting. This reduces any strain on the can that would
result from the air expanding during further heat processing and reduces the
possibility of the air oxidizing the inner can surface during storage.
Exhausting is carried out by:

- filling very trot,- cold filling and then putting the cans
with the lids loosely fitted into a steam chest or exhaust box,- blasting a
jet of steam into the headspace immediately before seaming.

The cans are then closed using seamers of the type shown
earlier. The next stage involves heat processing the sealed cans in boiling
water or in steam retorts.

Cooling

After processing for the required time the cans should be cooled
in clean chlorinated water. The cooler illustrated in Figure 3-4 in the section
on glass packaging has been found equally applicable for can cooling. The cans
are removed from the cooler while still warm as this allows them to dry quickly
and prevent rusting. They then pass on for labelling.

Can quality control

The double seam is the potential weak point of a can and for
proper hermetic seals it must be made to stringent tolerances. Routine
inspection of cans by 'tearing down' is important. Table 3-5 shows typical
tolerances for selected sizes of cans.

Table 3-5

Inspecting cans and adjusting the seamer is skilled work and
operator training is essential. Such training is usually provided by the can
supplier. It is not possible in a publication of this size to include full
details of can seam inspection and machine adjustment, this can be found in
special booklets, about 20 pages long, provided by can suppliers. The use of a
special micrometer, called a can micrometer, is necessary to measure the seam
width, seam depth and the cover hook and body hook. From these measurements the
% overlap of the two hooks can be calculated. The % overlap is the main factor
to maintain a hermetic seal. If necessary the canning machine is then adjusted
by increasing or decreasing the tightness of the first and second rolls. As
every change in roll tension made results in changes to all the other seam
dimensions this is a highly skilled job. Common seam defects are shown in Figure
3-18.

Figure

After filling and seaming an internal vacuum will form as the
hot contents cool. This internal vacuum is essential for preservation of the
contents and regular samples need to be taken and tested with a special can
vacuum gauge, again available from can suppliers.

Problems with poorly sealed or processed canned foods will
usually show up in store as 'swells' or 'blown cans.' Swelling takes place as
the food deteriorates and gives off gas. Instead of an internal vacuum the cans
are under pressure and if punctured the contents will blow out. The lids will
bow outwards due to the internal pressure; rather than inwards as in a can with
normal internal vacuum. Samples of finished stock in store should be routinely
checked for internal vacuum and any sign of blowing. If blowing occurs it is
often necessary to reject the whole batch.

Better stockroom control and response to customer complaints are
possible if the cans are date-coded. Small peddle-operated presses are available
that indent a series of numbers or letters into the lid before it is joined to
the can body.

Drums are made of sheet steel 0.4 to 1.5 mm thick which may be
galvanised and coated internally. They are strong and provide excellent
protection against light, moisture and rodents etc. As many drums are made for
use in the chemical industry it is important to check that any internal coating
is of 'food grade' quality.

There are two main types of steel drums: closed head or open
head as shown in Figure 3-20. Closed head drums are used for packaging liquids,
in particular edible oils, while open head drums find use for packing solid
products.

Figure

In many parts of the world second hand drums find use for bulk
packaging and distribution. It is very important that the food manufacturer
makes sure that these drums have not been used for dangerous chemicals.

The use of open-ended drums for storage of finished dried foods
is important. Such storage can provide good protection against pests, light and
moisture particularly if the drums are lined with plastic. Drums lined with
heavy plastic bags also provide good packaging for semi-processed ingredients,
for example fruit pulp preserved with sulphur dioxide or vegetables in brine.

Tins

Being totally impervious to light, air and moisture, tins
provide excellent protection. A large range of shapes and sizes, round, square
and cylindrical are available. Lids may be of the simple push-on type or hinged.
In many cases tins used for food packaging are attractively printed and have
great promotional and customer appeal. They also have appeal to the purchaser in
that they can be used for food storage in the home after use. Tins, and in
particular printed ones, are however an expensive form of packaging.

Two main types are of interest to the small to mediumscale food
manufacturer. Round tins with push-on lids are excellent for packing high-value
solids such as herbs. Round or square tins with a small pouring spout are
commonly used for packing cooking oils.

As round tins for dry goods are expensive compared to
alternatives such as plastic bags they are normally only used for packing
high-value products, particularly those that may loose flavour, odour or colour
if not well protected. Large manufacturers often pack such materials under an
inert gas (carbon dioxide or nitrogen) in order to protect them from oxidation.
Gas packing, as a technology, is generally considered to be beyond the means of
the small to medium producer. A low-cost system shown in Figure 3-21 has however
been successfully applied in trials for packing high-value herbs.

Figure

In use, the product is filled into the tin and the lid put on, a
tiny hole is then punched through the can base. Vacuum is applied to this hole
and the air drawn out. The vacuum valve is then closed and the gas valve opened.
This fills the can with gas. Finally a small drop of solder is applied to the
hole.

As in the case of drums, tins are often re-used. Again the food
manufacturer must assure themself that they have not been used for any toxic or
dangerous material.

Simple labour-intensive technologies for making cans of the
screw-on-lid type for vegetables oils at rates between 20 and 1000 per hour have
been developed and tested. The basic steps involved in making a round can with a
pouring spout for packaging oil are shown in Figure 3-22. It is not known how
much uptake there has been of this small-scale can-making technology.

Figure

In general smaller industries will fill tins by hand, possibly
with the aid of funnels. However overhead gravity fillers, piston fillers and
volumetric powder fillers, of the type described in Chapter 4 may be used.

Quality control for drums and tins

- Critical factors

- must not have been used for poisonous substances
if second-hand,- linings must be food grade.

- Major faults

- must not leak,- lids must seal,- no
internal corrosion.

- Minor faults

- dented.

3.1.4 Plastic bottles, jars, tubes, cups and trayes

Largely for cost reasons rigid plastic bottles, jars, tubes,
cups and trays are increasingly replacing glass and tin cans for food packaging.
Unfortunately the widespread use of plastic is having a bad impact on the
environment. Plastics do not rot or break down under the natural action of the
environment. They cause visual pollution floating in water or laying on the
ground and if burnt give off noxious and often toxic fumes. At the present time,
biodegradable plastics are not commercially available. With time it is hoped,
however, that safer, biodegradable plastics will be developed and, probably due
to pressure from legislation, replace the existing range of plastic packaging.

The range of plastics and co-polymers used to make rigid plastic
food containers is wide. In reality for most small food processors in developing
countries the choice will be restricted to packaging made of polypropylene,
polythene and polyvinylchloride (PVC). Polyethylene tetraphthalate (PET) is
however rapidly becoming more common. For the food processor plastic containers
have the great advantages of:

Plastic containers however give less protection than colored
glass and cans against light and air. In addition they are not as strong, in
terms of weight bearing and crushing, as glass or cans and are easily punctured
by sharp objects. It should also be remembered that rigid plastic packaging has
the considerable disadvantage of causing environmental since they are not
biodegradable. In general they cannot be easily re-used or re-cycled.

As is described later most plastic packaging cannot be used at
high temperatures so hot filling and heat processing are less common. If
high-temperature resistant polypropylene packs are not available then the types
of food that can be packaged at small scale into plastic are thus limited to:

- Foods that are naturally stable for the planned shelflife and
can tee cool filled (such as dried goods, some pickles, cooking oils, fats,
yoghurt, fruit juices containing preservative, beers, vinegar and honey).-
Jams and pasteurized pickles, such as chutney provided that the product is
cooled to below about 60°C before filling. In the case of jams this means
that a special recipe has to be used using a slow-setting pectin (a pectin that
does not set until the jam has cooled).

The commonest uses for rigid plastic containers are shown in
Table 3-6.

Container

Application

Plastic bottles

non alcoholic beverages,

cooking oils, ketchups,

sauces

Plastic jars

honey, spreads, peanut

butter, dry foods

Trays and tubs

butter, fats, spreads,ice

cream, jams, condiments

Cups

drinks, yoghurt

Tubes

honey, spreads

Table 3-6: Common uses for plastic containers

A wide range of different types and mixtures of plastics are
used to make plastic containers many of which are not suitable for contact with
food for they contain chemicals, known as plasticizers, that are toxic and can
migrate from the plastic to the foods. Oily foods are particularly likely to
dissolve plasticizers. The food manufacturer must make certain that the type of
plastic being used to make the container is food grade. It should be noted that
one particular type of plastic, PVC, is made in many grades only some of which
are food grade. In many countries regulations state which types of plastic can
be used and local Standards Offices can advise. In cases where no such standards
exist the recommendations of countries with established standards should be
consulted.

Production of plastic containers

Plastic bottles are made by several methods:

- Blow moulding is similar to glass bottle making and is used as
a one or two stage process to make bottles, jars and pots- Injection
moulding. Here grains of plastic polymer are heated by a screw in a moulding
machine and then injected under high pressure into a cool mould. The method is
mainly used for wide necked containers and lids.- Injection blow moulding.
Polymer is injection moulded around a blowing stick and, while molten is
transferred to the blowing mould. It is then blown into shape by compressed air
(Figure 3-23).- Extrusion blow moulding. In this method a continuously
extruded tube of softened polymer is trapped between the two halves of a mould.
It is then inflated by compressed air into the mould.- Stretch blow
moulding. A shape is prepared by either injection or extrusion moulding. It is
then re-heated, which causes the molecules of plastic to 'line up'. This gives a
glass clear container of greater strength which has good barrier properties to
gases and moisture over a wide temperature range.

The costs of moulds for injection moulding are much higher than
those for extrusion moulding, but the surface finish and size accuracy of the
finished product is better. It is possible, by injecting two or more different
types of softened plastic, one inside each other, to produce bottles with layers
of different types of plastic. These are called co-extruded bottles and are used
to give special properties such as improved gas permeability characteristics.

Tubs, trays and cups are made by heating sheets of thermoplastic
material and then shaping the soft sheet into a mould by means of vacuum or
pressure. While such packaging is normally made in large factories, smallscale
semi-automatic vacuum thermoforming machines are available (Figure 3-23). Such
small-scale local production of plastic containers could offer opportunities for
entrepreneurs in developing countries.

Figure

The types of plastic commonly used for food packaging materials
likely to be available in developing countries are shown in Table 3-7.

Table 3-7

In addition to the above, plastic packaging is made by combining
different plastics or co-forming them to improve, for example, the packs water
vapour or gas barrier properties. In this way the materials used in the
coforming can be tailored to the product. In some cases up to six different
layers of material may be used.

Selection of best material for a product

Ideally the type of plastic used for a particular packaging
application should be selected with advice from the supplier. The reality for
many small-scale food manufacturers however will be that only one or two types
of bottle are available. The food producer should find out what type of plastic
containers are available and then carefully consider aspects such as:

- Is the plastic suitable for contact with food.- Is
resistance to oils and fats important.- Strength, particularly if gassy
drinks are involved.- Is permeability to gases (oxygen and carbon dioxide)
important.- Maximum filling temperature that can be used.- Color,
clarity and surface finish.- If hand capping can be used or if special
closing machines are involved.- If heavier grade, stronger shipping
containers will be needed to protect against crushing and impact damage.- If
plastic cups are to be used considerable transport savings can be made by
selecting types that stack one inside the other. It is possible to pack 8700
conical stacking cups per cubic metre but only 1500 straight sided ones.
Transport cost savings of over 80% could thus be made by using conical cups.

Processing, filling and sealing

Plastics, with the exception of polypropylene, have poor
resistance to high temperatures; In general then packaging cannot be hot water
or steam sterilized before filling. Thorough washing in clean water to remove
any dust is thus essential followed by draining.

If OPP or HDPE are being used hot filling is possible. In the
case of other plastics filling temperatures will need to be kept below
60°C. At a small scale, hand-filling will commonly be used but of course
piston, vacuum and gravity fillers as described in Chapter 5 can be appropriate.
Closing or sealing plastic packaging depends on the type being used.

Bottles and Jars.

The most common closures used are the same as those used for
glass bottles. These include plastic screw-on caps that may if desired be
pilferproof (ROPP) and hinge up/snap down plastic caps that are increasingly
being used for products, such as oils, that are frequently opened and closed. In
some cases these hinge up caps are fitted with a small pouring spout, very
convenient to the customer for sauces and honey. Caps of these types and small
equipment for applying them are described in Chapter 3.1.1 on glass packaging.
Plastic shrink sleeves and aluminium foil capsules can be slipped over the cap
to make it pilferproof and more attractive. Such sleeves are also described in
Chapter 3.1.1 on glass.

Tubs and cups.

Containers of this type are closed in two ways: with snap-on or
push-on plastic lids or heat sealed aluminium foil. At a small scale,
hand-closing is invariably used. If heat sealed lids are to be used then the
container must have the correct shaped rim to which the foil lid is sealed. This
has an electrically operated sealing head operating at about 200 °C and can
seal 10 pots/minute. As the thin foil closure can easily be damaged some
manufacturers place a snap-on lid over it to give added protection

In operation the filled cup is placed onto the platform and a
foil lid is laid onto the cup rim. The heat sealer head is brought down for a
set time and the plastic on the back melts and heat seals onto the rim. It is
possible, if heat sealers of the type above are unavailable or too expensive, to
use a household domestic iron to heat seal foil to cups as shown in Figure 3-26.
Several locally made sealers made this way have been seen. The main problems in
using them are obtaining the correct sealing temperature and time. This must be
found by trial and error.

Figure

Plastic tubes.

While plastic tubes are not commonly used for food packaging in
developing countries they are an option for food manufacturers who wish to sell
their products with greater customer convenience. Applications include honey,
mustard and sauces. Tubes are purchased with the neck and cap complete, the
bottom end being open. The product is filled into the open bottom, taking great
care to keep the part that is later heat-sealed completely clean (Figure 3-27).
Piston fillers are very useful for such filling. After filling the open end of
the tube is heat-sealed in a jaw sealer of the type described in Chapter 3.2.2
on films.

Figure

After filling, sealing and labelling, plastic containers should
be packed into outer cardboard shipping boxes. While it is best to use cardboard
dividers in the boxes it is not as important as when using glass for plastic is
not subject to impact damage. It should be remembered that plastic is not as
strong as glass or cans and so care is needed not to stack boxes too high to
avoid crushing. This is particularly true of products, such as yoghurt, packed
in cups. Outer cases should then be sealed, preferably labelled and date-stamped
to make stockroom control easier.

Quality control and special skills needed

The main quality control procedures needed when using rigid
plastic packaging are those general methods described in Section 6.4 of this
publication (net weight, shelf life etc). Because plastic packaging is light in
weight variations will cause considerably less final net weight control problems
than when using glass.

If heat-sealed foil is being used it is important to carry out
frequent checks for proper sealing. Packs should be turned upside down to make
sure they do not leak and checks made by filling with warm water and sealing so
that an internal vacuum forms after cooling. This can be easily seen as the foil
will bow inwards.

No specialized skills that cannot be learnt in the plant are
needed for filling and sealing rigid plastic packaging at a small scale.

3.1.5 Wooden containers

While wood is widely used for packaging fresh produce its use is
limited when dealing with processed foods. The most common applications are:

- barrels for wines, beers, spirits, salted fish and vegetables
in brine,- wooden crates, particularly for bottles that are returnable,-
tee chests,- small fancy boxes for foods aimed at a tourist or gift
market,- to construct pellets.

Wood is strong and provides better protection against crushing
and impact than cardboard boxes. It is however heavier and more expensive. Wood
containers can be made lightproof and leakproof. As a material wood is porous
and so does not form a perfect barrier to moisture and air. Depending on the
method of construction wood containers can provide excellent protection against
pests.

Barrels are very difficult to make and the training takes
several years. They are also very expensive and so are re-used over and over
again' being sent back to coopers to repair any damage. They are available in
many sizes from less than 5 gallons up to huge barrels that contain a tonne or
more of product. The common sizes however are light enough for a person to lift
or move.

If barrels are to be considered as a packaging for foods the
following points need to be borne in mind:

- they must be returnable, deposits should be charged,- care
should be taken if buying second-hand that no contamination, for example the
odour of fish, is possible,- the food producer should have space and
facilities for thorough washing and cleaning and workers skilled in minor
repairs, fitting the wood lids and tightening the metal barrel hoops.

Many small food manufacturers use wooden crates to distribute
food in bottles to shops, particularly when the bottle is returnable. Such
crates, which usually hold 24 packs, can easily be made by local carpenters and
Figure 3-28 shows a simple jig being used to greatly speed up production.

Figure

If distribution of products that might suffer from crushing, for
example yoghurt or foods in plastic bags, is considered then crates used should
have 'stacking corners' as shown in Figure 3-29.

Figure

It is recommended that some form of permanent owners mark
-painted or burned in -is made on delivery crates to make sure they are indeed
returned.

Tea chests are a very special case where a wood packaging has
become the accepted standard all over the world. They are made of thin plywood
over a timber frame and corners and edges are bound with tin strips to give
protection against dropping. Tea chests are lined internally with a paper/foil
laminate which provides an excellent moisture and air barrier. The only real
application of tea chests is for bulk distribution and export of tea.

The use of small wood boxes, for packaging goods for the tourist
and gift market can, in certain cases, provide opportunities for small food
manufacturers. Generally the containers will be supplied by a local craft group
or carpenter. They are ideal for dry goods such as spices and herbal teas
although an inner plastic bag would always be recommended to give better
moisture protection and avoid the chance of wood splinters entering the food.
Some producers market a range of local foods in an open-topped box over wrapped
with cellophane.

Few small or medium-scale food producers are likely to
distribute final products on pallets but their use is strongly recommended in
order to hold finished products in store off the ground. They can easily be
built by local carpenters, the simplest design being shown in Figure 3-31. It
can easily be seen how such a pallet keeps the food off a possibly damp floor
and also allows easy cleaning of the storeroom.

Figure

3.1.6 Paperboard

Paperboard is the general name given to a variety of different
types of materials that are used to make boxes, cartons and trays to package
foods. They can be used as shipping (outer) containers or as consumer packs, but
only a few types of materials can tee used directly in contact with foods.

In this section the different boards are first described.
Corrugated boxes are dealt with in more detail because these are among the most
common types of shipping container available to small processors. It should be
noted that paperboard packs can be designed, made up, printed and sealed by the
processors and are therefore one of the few packaging systems that are within
their control. The methods for doing this are described in some detail in this
section and are also included in Chapter 5.

Paperboard is produced in the same way as paper (section 3.2.1)
but it is made thicker and often in multiple layers, to protect foods from
mechanical damage (crushing, puncturing, vibration). There is a large range of
paperboard types for different applications as consumer packs or shipping
containers. Cartons or boxes are printed (if necessary), cut out to the
appropriate size and shape and creased. The flat carton (or 'blank') may then be
glued and assembled by the board manufacturer or alternatively delivered to the
food processor for assembly on site. Types of paperboard are discussed in the
following sections.

Moulded Paper Packaging

A number of packaging materials are made from recycled waste
paper. The most common of these are egg trays and egg boxes but others such as
fruit trays, small shallow dishes and protective bottle cases are available.

Moulded paper packaging (MPP) is mainly produced at very large
scale but technology has been developed to produce such items at medium scale,
which is still too expensive for small or medium producers. Recently the
technology has been further scaled down to a level that is within the reach of
small entrepreneurs at a cost of approximately 12,000 and with an output
of 240 trays per hour .

The first step in making MPP is to prepare paper pulp by
liquidizing paper in water. Printed paper should not be used if the packaging is
to come into direct contact with food. If necessary colours may be added or, if
a degree of waterproofing is necessary, waxes.

Moulding takes place on a two-part mould, a forming mould and a
transfer mould. The forming mould is made of fine wire mesh and the transfer
mould of plaster like material. Vacuum and compressed air are supplied to the
moulds. The process involves dipping the forming mould into the paper slurry so
sucking up a coating of fine fibres of paper. Compressed air is then used to
blow the formed item off the transfer mould. After moulding the trays are very
wet and have to be dried, usually in the sun.

Paperboard that is used for fibreboard (more commonly named
cardboard) boxes has no coating and as a result the barrier properties to air
and moisture are low. Cardboard boxes are widely used as shipping containers for
almost all foods. The properties of cardboard can be improved by a coating of
wax or by lamination with polythene for use in consumer packs (for example
paperboard cartons). These are used alone for products such as salt, rice, pasta
products and spices or to provide protection against mechanical damage, or for
inner plastic or paper bags containing a wide range of foods such as cereal
products, snackfoods, coffee and confectionery. The main types of paperboard
that are used for foods are described below. It should be noted however that
there are many variations on these basic types and a large number of tradenames,
particularly for specialist boards that have specific properties. A
comprehensive list of these boards is not included in this publication. Board
thickness is one of the main considerations and the figures below are the weight
of board per square metre which is a measure of the thickness (higher weight =
thicker board).

White board

This is the only type of paperboard that is recommended for
direct contact with foods. It is made from several thin layers of bleached
chemical pulp and it is usually used as the inner layer of a carton combined
with other types of board which form the outer layers of the carton. It may be
coated with wax or laminated with polythene to enable it to be heat sealed.

Triplex board (or foodboard)

This is widely used for food packaging. It normally has three
layers, the inner and outer layers being made from white board (bleached
chemical pulp). The outer layer may be machine glazed and/or coated to enable a
better print quality to be achieved and it is supplied as 200 400
g/m2. Another board named Duplex board (or boxboard) is similar but
the inner layer is made from grey (ie unbleached) chemical pulp.

Chipboard

This is made from re-cycled paper and is used to make the outer
cartons for packets of foods such as tea and cereals. It is not suitable for
direct contact with foods. It is less strong than Triplex or Duplex board for an
equivalent thickness but it is cheaper than these materials. It is usually
supplied as 300 g/m2 and it may be lined with white board to improve
its appearance and strength

Solid board

This is a multiple layer of bleached sulphate board that is
white, strong and durable. It is usually supplied as 150 - 400 g/m2
and when laminated with polythene it is used for liquid cartons (sometimes named
Liquid Packaging Board or Milk Board). Examples of typical products packaged in
liquid packaging board include fruit juices and soft drinks.

Fibreboard

This can be either solid or corrugated board The solid type has
an outer kraft layer and an inner white board It provides good protection
against impact and compression and is often spirally wound into cylinders or
small tubs which are fitted with a plastic or metal cap at the top and a board
inserted at the base. A composite can is a can-like package with the body and
ends made of different materials. The body is usually made of paper and then
ends of metal and increasingly plastic. The body is made from spirally wound
paper in a tube shape. Better barrier properties are obtained if the paper is
laminated with plastic or aluminium foil. Composite cans may, rather like
sanitary cans, be bought plain or printed with the base fitted or for total
assembly in the factory. They mainly find use for packing dry goods including
coffee, cocoa, ilk powder and mustard powder. They are cheaper than metal tins
and can be made from re-cycled paper. Small containers are used to package
spices or confectionery for retail sale and larger drums are used to package a
variety of powders and dried foods for shipping and distribution.

It is important that these containers are kept dry at all times
and not stored in a humid environment to avoid delamination and loss of
integrity of the drum.

Corrugated cardboard (or fibreboard)

This is made with two layers of kraft paper and between them, a
central corrugating (or fluting) material. The corrugations are made by
softening the fluting paper with steam and then passing it over corrugating
rollers. The kraft paper is then glued to each side. Thicker boards may have
several layers of corrugated board glued together, although these are not widely
used in foodapplications. The best quality board has unbleached kraft paper
with equal thickness either side of the fluting and uses no re-cycled material.
Both bleaching and the use of re-cycled paper reduces the strength of the
cardboard.

The degree of protection from mechanical damage that is provided
by cardboard depends on the size and number of corrugations or 'flutes'. Smaller
more numerous corrugations give rigidity to resist compression from stacking,
whereas larger corrugations give a cushioning effect that resists impacts and
puncturing.

Corrugated boards resist impact, abrasion and crushing damage
and are therefore widely used for shipping containers for bulk foods such as
dried fruit, nuts, etc., and for containers such as glass jars and plastic films
that require protection. They are also used to contain cans and plastic tubs or
bottles for convenient handling. An alternative to boxes is shrinkwrapping or
stretchwrapping (Section 3.2.2) which, if available, gives a more sophisticated
image. In many countries however these remain more expensive than cardboard.

Supply of boards

Dimensions of boxes and cartons are always quoted in the order:
length x width x height and the dimensions are always the inside measurements of
the box taken from the centre of a crease to the centre of the next crease
(Figure 3-34). Designs of cardboard box are shown in Figure (3-35).

Figure

Figure

The board can be supplied in a number of ways to smallscale
processors. The most convenient, but also the most expensive, is to receive the
board already formed into blanks. Here the board is cut to the correct shape and
scored along the folding lines so that it can be easily assembled in the
production area (Figure 3-36). It can also be supplied pre-printed.

Figure

Alternatively the board can be supplied as plain sheets which
must be cut to shape, scored and folded by the processor. This is more
time-consuming and requires a separate preparation area away from the food
production to prevent contamination of the food with dust and fragments of card.
Additionaly the processor is paying for the waste card that is not used (Figure
3-37). However in many countries where large packing boxes are available,
perhaps from imported equipment, the supply of corrugated cardboard sheets is a
suitable way for smallscale processors to solve their packaging problems by
producing their own boxes. Methods for doing this are therefore described below.

Figure

All cardboard boxes should be carefully stored, especially in
humid conditions to prevent deterioration of the material and separation of the
corrugations or delamination of the layers of a package. This depends on both
the type of adhesive that is used to seal the board and the conditions under
which the containers are stored and handled. In general they should be kept dry,
cool and off the ground on pallets or shelves.

Calculation of box size

The factors to consider when deciding on a cardboard box for
packaging foods are the box size required to adequately contain the contents and
the most economical box shape. The size of box required can be found by placing
together the containers to be packed (not forgetting dividers if these are to be
used between jars or bottles) and measuring the size of the stacked food (Figure
3-38). These sizes are then the minimum internal dimensions of the box. In
practice it is usual to then choose the nearest standard size of box that is
supplied, rather than pay the extra cost of a specially made box. Care is needed
that there is not too much free space inside when it is full. The containers
should be firmly held in place to prevent them from moving and being damaged
during transport.

Figure

The most economical design which minimises the amount of board
used to make a box of a given volume is found when the ratio of
length:width:height equals 2:1:2. This is because less card is used to form
overlapping flaps compared to other designs where the ratio is different (Figure
3-39).

Figure

Sealing

The most common type of adhesive used for gluing cardboard boxes
is based on starch (usually cornstarch) which is specially treated for hot,
humid conditions to make it more resistant to moisture pickup and consequent
weakening of the bond. Boxes may also be stapled (Figure 3-40) or occasionally
stitched. After filling the boxes may be sealed using glue, staples or stitches
as above. Glues should be fast-setting (for example polyvinyl acetate glues) to
ensure that the cardboard flaps stay in place. Alternatively they may be tied
with string/rope or taped or strapped with tape (Figure 3-41). Simple tape
applicators are available which make sealing faster and more economical.

Figure

Figure

Quality control tests for paperboard

In practice most small-scale producers have only one supplier of
boxes or cartons and in some countries the only supply is recycled used boxes.
In these situations it is unlikely that any action can be taken if the quality
of boxes falls below specification. However, for those producers that have a
choice of supply it is worth monitoring the quality of boxes and cartons and
ensuring that the supplier understands the needs of the processor. Specific
tests for paperboard are described below and more general quality control
considerations are described in Chapter 6.4.

The operators in a food processing unit can check the
appearance, print quality, etc., of cartons and boxes by looking for these
faults on a routine basis. If a problem arises then the dimensions of the boxes
can easily be measured with a rule and the position and depth of score lines can
be checked. No other special quality control equipment is
needed.